1. Field of the Invention
The invention relates to an enhancement mode high electron mobility transistor (HEMT).
2. Description of the Related Art
For AlGaN/GaN HEMTs, it is well-known that enhancement mode (normally off) HEMTs are superior to depletion mode (normally open) HEMTs.
FIG. 1 shows an enhancement mode AlGaN/GaN HEMT including a P-type GaN-doped gate electrode 11. However, the doping process thereof is complex.
FIG. 2 shows an enhancement mode HEMT including a Schottky-type source electrode 61. However, the threshold voltage of the enhancement mode HEMT is difficult to control, and the produced tunneling current limits the maximum saturated output current of the HEMT.
FIG. 3 shows a field plate 12 to improve the breakdown voltage of HEMT. However, the introduction of the field plate increases the parasitic capacitance of the HEMT, which adversely affects the high frequency and switching properties thereof. For example, low concentration of fluoride ions treatment technology in the barrier layer 10 between the gate and drain is adopted. However, the fluoride ions treatment induces damage to the AlGaN/GaN heterojunction; moreover, the distribution of the fluoride ions in the thin barrier layer is difficult to control.
FIG. 4 shows a layer of GaN 13 on the barrier layer to improve the breakdown voltage of HEMT. However, when the gate voltage is large, leakage current is produced in the gate electrode, which limits the gate voltage swing.